Electrodeless discharge lamps operate by transmitting a high-frequency electromagnetic signal into a transparent sealed vessel containing a gaseous mixture, typically a mixture of mercury vapor and an inert gas. The electromagnetic energy creates a plasma of circulating charged particles, which excites the mercury atoms to higher energy states. When the mercury atoms fall back to their normal energy state, they give off light radiation, mostly in the non-visible, ultraviolet portion of the spectrum. The ultraviolet radiation impinges on phosphors that are coated on the surface of the vessel, and these phosphors in turn emit visible light.
The electromagnetic signal is generated by an induction coil, which is driven by a high-frequency amplifier whose output is preferably fed through a filter and matching network. Circuitry for driving the induction coil is described in commonly-owned patent application Ser. No. 07/887,168 now U.S. Pat. No. 5,306,986 and patent application Ser. No. 07/955,528, now abandoned, both of which are incorporated herein by reference.
Electrodeless discharge lamps are highly efficient, providing an output of approximately 60 lumens/watt, as compared with approximately 15 lumens/watt for a normal incandescent light bulb. These lamps therefore offer the prospect of substantial energy savings. These energy savings will be further enhanced if users of the lamps are able to adjust the intensity of the light output of the lamps to meet their needs. Arrangements according to the broad principles of this invention allow this to be done.